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Descriptions

Disease control of soilborne pathogens by biological control agents
(BCAs) is often inconsistent under field conditions. This inconsistency
may be partly influenced by pathogen diversity if there is a differential
response among pathogen species and isolates to selected BCAs. The
responses of 148 Pythium isolates obtained from soil at three forest
nurseries and representative of 16 Pythium spp. were evaluated in the
presence of Streptomyces lydicus strain WYEC108 in an in vitro assay.
Percent growth inhibition, inhibition zone distance, mortality, and
growth rate were recorded for each isolate, and data were analyzed for
effects of species and isolate. Responses of three Pythium spp.
(Pythium irregulare, P. sylvaticum, and P. ultimum) were further analyzed
for a location (nursery) effect. Although S. lydicus inhibited all
Pythium isolates, differences in percent growth inhibition, inhibition
zone distance, and mortality were observed among Pythium spp. and
isolates. Small but significant location effects were also noted. Growth
rate also varied among Pythium spp. and isolates and was found to
strongly bias percent growth inhibition and, to a lesser degree, inhibition
zone distance; depending on which measure was used, slower-growing
isolates appeared less sensitive (growth inhibition) or more
sensitive (inhibition zone) to S. lydicus than faster-growing isolates.
Results illustrate the importance of using multiple, representative pathogen
isolates in preliminary BCA inhibition assays as well as accounting
for the effect of pathogen growth rate on pathogen inhibition by
BCAs. Future studies should take pathogen diversity into account
when evaluating biological control efficacy.

Financial support was provided, in part, by funds from the United States Department
of Agriculture–Agricultural Research Service CRIS Project 5358-12220-004-00, and grants from the Pacific Area Wide Pest Management Program for Methyl Bromide Alternatives.